1. Field of the Invention
The present invention relates to light emitting elements and a manufacturing method thereof. More particularly, the present invention relates to light emitting elements as exemplified by laser elements using nitride semiconductors, and to a manufacturing method thereof.
2. Description of Related Art
Being compounds between a group-III element, such as Al, Ga or In, and N, a group-V element, nitride-based semiconductors (for example, AlN, GaN and InN, including solid solutions of these, such as AlGaN and InGaN-in the present specification all these are collectively called nitride semiconductors) are, for their band structure and chemical stability, expected to find applications as materials for light emitting elements and power devices.
On the other hand, nitride-semiconductor elements are widely used that employ a sapphire or GaN substrate, which has a hexagonal crystal structure, and that have a layered structure of nitride semiconductors formed on the c-plane ((0001)-plane) of the substrate. In particular in applications as laser elements and as light emitting elements such as in light emitting diodes, some nitride-semiconductor elements employ, as an active layer, a quantum well structure composed of GaN (barrier layers)/InGaN (well layers). Here, increasing the In content of InGaN permits the light emission wavelength to be varied to the longer wavelength side.
Increasing the In content, however, increases the strain due to lattice mismatch between GaN and InGaN, and thus enlarges the piezoelectric field. As the piezoelectric field augments, the overlap between the wave function of electrons in the conduction band and the wave function of holes in the valence band diminishes, and thus electrons and holes in pairs are spatially separated from each other. This results in problems of increased threshold current and lower light emission efficiency. Moreover, since a nitride semiconductor having a wurzite structure, a hexagonal crystal structure, has no symmetric planes with respect to the c axis direction, polarities appear in a nitride semiconductor grown in the c axis direction, and thus spontaneous polarization occurs, another factor leading to the problems.
In order to address these problems, Non-patent Document 1 proposes a light emitting element in which a nitride semiconductor layered structure is formed on the m plane ({1-100} plane), which is a non-polar plane, of a GaN substrate, and in which a ridge stripe is formed in the a axis (<11-20> axis) direction or in the c axis direction. Also by forming a nitride semiconductor layered structure on the a plane ({11-20} plane), which also is a non-polar plane, of the GaN substrate, it is possible to reduce the piezoelectric field and thus spontaneous polarization.
Non-patent Document 1: Japanese Journal of Applied Physics, Vol. 46, No. 9, 2007, pp 187-189.
However, it is difficult to form a nitride semiconductor layered structure on the m or a plane of a substrate as compared with forming it on the c plane, resulting in a problem of lower yields. Since a nitride semiconductor laser element emits light with a short wavelength and high energy, its resonator facets are prone to be damaged, another factor leading to lower yields. In order to improve the yield, to surely prevent, among others, COD (Catastrophic Optical Damage), which is the cause of lower light emission strength and of damaged facets, becomes a main issue. In nitride semiconductors in particular, oxidation of facets may greatly affect the occurrence of COD.